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Scout Now For Soybean Insects

South Dakota State University | Aug 01, 2003

SDSU Extension Entomologist Mike Catangui says soybean aphids, bean leaf beetles and grasshoppers are showing up in soybean fields throughout South Dakota. Insect infestations at this time of year can result in serious yield losses because soybeans are now in the pod-fill stages.

"Soybean plants are the most sensitive to injury right now," Catangui says.

Soybean aphids: An average of 200 soybean aphids per plant is currently considered economically damaging. Soybean aphids are tended by ants which "farm" the aphids for their honeydew. Scout for them by watching for ants along the borders of soybean fields as one sign that aphids may be present.

A new pest to South Dakota since 2001, soybean aphids are very small but can be seen with the naked eye. They are about one-sixteenth of an inch long and yellowish to yellowish-green in color. A magnifying lens will also reveal that they have a pair of black "tail pipes" on the rear. They're found on the growing points, stems and on the underside of leaves.

SDSU research in 2002 conducted by Catangui at the Southeast Research Farm near Beresford has shown that spraying for the soybean aphids during R5 (beginning seed) stage in early August can improve yield by 2-11 bu. (8-27%)/ acre, depending on the insecticide and rate used.

Bean leaf beetles can be very destructive pests of soybeans because they feed directly on the pods and developing seeds. Whole pods may also fall to the ground as a result of pod clipping by the beetles.

SDSU research in 2002 conducted by Catangui on a grower's field near White has shown that spraying for the bean leaf beetle during R5 (beginning seed) stage in early August can improve yield by 2-9 bu. (7-27%)/acre depending on the insecticide and rate used.

Grasshoppers: The economic threshold at which it pays to spray for grasshoppers is defoliation, or leaf loss, of 20%. Twenty percent leaf loss means that 20 out of 100 of similar-sized soybean leaves completely eaten by the insects.

SDSU research in 2001 conducted by graduate student Dave Mills at the Southeast Research Farm near Beresford indicated that a 50% defoliation at full pod stage soybean can result in 8 bushels per acre (15%) yield loss.